This invention relates to earth drilling techniques in general and, in particular, to a new and useful method and apparatus for producing fractures or gaps in geological formations of crystalline rock in great depths for utilizing the heat of the earth, whereby, one or more bores are sunk into a suitable rock, which is put under a specific pressure above its breaking limit by means of a pressure medium so that a large-surface fracture or gap is formed through which a heat carrier is subsequently conducted.